This invention relates generally to gas turbine engines and more particularly to a turbofan engine having a duct burner.
Some new aircraft designs under study have diverse propulsion needs in terms of sea-level static (SLS) take-off thrust levels, cruise thrust and specific fuel consumption (SFC) levels, engine diameter and length restrictions, and exhaust system shaping (i.e. for “low observables” purposes) that make it difficult to utilize an existing engine or even to define a new non-augmented engine that meets all of these needs. In particular, demanding “hot day”, high altitude, short runway requirements can be difficult to meet with unaugmented or “dry” versions of existing engines that also meet applicable fan diameter limitations. Augmented (afterburning) versions of these existing engines are typically too long and are not as amenable to special exhaust system shaping as non-afterburning versions. Non-augmented, higher fan pressure ratio engines can be defined that supply the needed thrust within the diameter and length constraints but they will have higher than desired cruise segment SFC levels.
What is needed in such situations is an augmentation system that can supply a modest (e.g. about 15-25%) increase in take-off thrust for either existing engines, such as low-bypass military turbofan engines, or can be incorporated into a new engine design so a more optimum cruise cycle can be utilized. This augmentation concept must also be compatible with the length and shaping needs of the exhaust system.
Current engine augmentation systems are located in the engine tailpipe downstream of the rear frame. They can easily be sized to provide well in excess of the 15-25% thrust increase mentioned above, but will add appreciable length as well as not being highly adaptable to exhaust system shaping
“Duct burners”, i.e. augmentation systems placed in a bypass duct of an engine, have been demonstrated in the prior art. However, these duct burners required two separate exhausts, one for the primary stream and one for the augmented bypass stream, increasing the weight, complexity, and cost of the engine.